I don't think it is possible to use the 360's controller on the original xbox. The original xbox has no way of using wireless controllers unless they have a dongle. I believe you'd have to make a custom dongle for your controller, which is near next to impossible. I don't believe you can use a USB 360 controller either.
Also, using a PS2 controller (with an adapter) on the xbox has not worked for me. You could try it, though.
Amazon has used first party S controllers, for about 10 bucks. I'd buy one off there, ebay, or try to find a local used videogame store. A store in my area sells used controllers for 7 dollars, so look around.
http://www.amazon.com/Controller-S-Xbox/dp...duckduckgo-d-20As always, thanks for posting on the forums.